1. Field of the Invention
The present invention relates to a low cost optical fiber cable in which an optical fiber core having an outer covering is suppressed from movement inside the outer covering in the lengthwise direction thereof and which does not cause deterioration of the transmission characteristics even when the cable is bent, and to a method of producing the same.
2. Description of the Related Art
An optical fiber core is accompanied by a problem in that the transmission characteristics are deteriorated or the life is shortened when a tension larger than a predetermined value is applied thereonto. Therefore, an optical fiber cable accommodating the optical fiber core has been contrived so that a large tension will not act on the optical fiber core even when the optical fiber cable is laid, even when the optical fiber cable receives a stress due to wind pressure after the cable has been laid, or even when the optical fiber cable expands or contracts due to a change in the environmental temperature.
Usually, the optical fiber core covered with an outer covering has an excess length slightly larger than the length of the cable, so that no tension is applied onto the optical fiber core even when the optical fiber cable is expanded by the application of tension, provided the expansion is within the amount of the excess length.
Further, when the optical fiber cable is left to be laid for extended periods of time, the optical fiber cable receives vibration or undergoes expansion or contraction due to a change in the environmental temperature, whereby the optical fiber core moves in the cable in the lengthwise direction, the optical fiber core inside the cable protrudes beyond the end of the outer covering of the optical fiber cable, or the optical fiber core draws back into the outer covering.
For the purpose of preventing such phenomenon, in order that the optical fiber core will not freely move in the lengthwise direction in the outer covering, application of a jelly blend to the periphery of the optical fiber core has been attempted to increase the friction and allow only minimal movement of the optical fiber core. Or, it has been attempted to fix the optical fiber core to the outer covering so that the optical fiber core will not move at the end or inside of the optical fiber cable.
An example of the optical fiber cable according to the above related art has been disclosed in Japanese Laid-Open Patent No. 166733/1997. FIG. 3 is a view illustrating the above optical fiber cable, wherein FIG. 3A is a vertical cross-sectional view thereof, and FIG. 3B is a lateral cross-sectional view thereof as viewed in the X-direction. In FIG. 3, further, reference numeral 16 denotes an optical fiber core, 17 denotes tension members,18 denotes an accommodating member, 19 denotes movement-suppressing members, 20 denotes core wrap tapes, and reference numeral 21 denotes an outer covering. The names of the portions denoted by reference numerals comply with the terms used in this invention.
In this optical fiber cable, a plurality of optical fiber ribbons 16 are accommodated having an excess length in a slackened manner in a groove, which is in a U-shape in cross section, of the accommodating member 18 in which two tension members 17 are buried in the lengthwise direction, and the movement-suppressing member 19 composed of an adhesive or the like is provided so as to fill the surrounding of the optical fiber cores 16 in the groove of the accommodating member 18 intermittently in the lengthwise direction to secure the optical fiber cores 16 to the accommodating member 18. The accommodating member 18 is surrounded by the core wrap tapes 20 and the outer covering 21 to obtain an optical fiber cable.
In the optical fiber cable shown in FIG. 3, the optical fiber cores 16 are secured to the accommodating member 18 by the movement-suppressing member 19 composed of an adhesive or the like intermittently in the lengthwise direction. Therefore, the protrusion of the optical fiber cores 16 beyond the end of the outer covering 21 at the end of the optical fiber cable, or drawing back from the end of the outer covering 21 does not occur.
However, since the optical fiber cores 16 are secured to the hard accommodating member 18 by the movement-suppressing members 19 intermittently, when the optical fiber cable is bent at a small radius of curvature, an excessive tensile force is applied onto the optical fiber cores 16 near the movement-suppressing members 19, and a transmission loss of the optical fiber cores often increases by 0.1 dB or more.
If the optical fiber cores are not secured by the movement-suppressing members, the transmission loss of the optical fiber cores does not increase even when the optical fiber cable is bent at a small radius of curvature. However, the optical fiber cores are no longer suppressed from freely moving in the accommodation member in the lengthwise direction. Therefore, when an extended period of time passes from the laying, the optical fiber cores move in the accommodating member in the lengthwise direction as a result of expansion and contraction of the cable due to a change in the environmental temperature and vibration caused by wind pressure, etc. Consequently, the optical fiber cores protrude beyond the end of the outer covering at an end of the optical fiber cable, or draw back from the end of the outer covering.
In addition, the optical fiber cable shown in FIG. 3 is expensive since it uses the custom made accommodating member having a U-shaped groove.
This invention is aimed to provide an optical fiber cable at a reduced cost and a method of producing the same, without deteriorating the transmission characteristics even when the optical fiber cable is bent.
The optical fiber cable of this invention features buffer members composed of a long fiber longitudinally provided or stranded around at least one optical fiber core, and said buffer members and said optical fiber core are adhered together intermittently in the lengthwise direction, and an outer covering is provided to surround buffer members.
According to the optical fiber cable of the invention, the optical fiber core is suppressed from moving in the cable, and the transmission loss does not increase even when the cable is bent.
That is, in the optical fiber cable of this invention, buffer members are longitudinally disposed or stranded around the optical fiber core, buffer members and the optical fiber core are adhered together intermittently in the lengthwise direction, and an outer covering is provided to surround the buffer member. Therefore, even when the cable is expanded or contracted due to vibration caused by wind pressure, etc. or due to a change in the environmental temperature, after the cable is laid, the optical fiber core does not move in the lengthwise direction of the cable, and does not protrude beyond the end of the outer covering or does not draw back from the end of the outer covering.
Further, buffer members and the outer covering are also adhered together intermittently in the lengthwise direction to enhance the effect for suppressing the movement of the optical fiber core.
Further, in the optical fiber cable of the invention, the optical fiber core is protected by a material having a low bulk density, i.e., by buffer members comprising a long fiber. Therefore, even when the cable is bent, the bending stress is not transmitted to the optical fiber core, and the transmission loss of the optical fiber core does not increase even when the cable is bent.
In addition, owing to buffer members, the optical fiber cores are not damaged even when peeling off the outer covering for branching the cores. Accordingly, the operation can be conducted without any anxiety. Further, buffer members are cheaper than the accommodating member that is used in the related art, and the cost of the cable can be lowered.
According to this invention, the optical fiber cable is produced by applying an adhesive onto at least one optical fiber core intermittently in the lengthwise direction thereof, longitudinally disposing or stranding buffer members composed of a long fiber thereabout, and providing an outer covering to surround buffer members.
Further, according to this invention, the optical fiber cable is produced by longitudinally disposing or stranding buffer members composed of a long fiber around at least one optical fiber core, dropping an adhesive onto the surface of buffer members intermittently in the lengthwise direction so as to exude into the peripheries of the optical fiber cores passing through the gaps in buffer members, and providing an outer covering to surround buffer members.
To adhere buffer members and the outer covering intermittently in the lengthwise direction, the adhesive applied onto the optical fiber core is caused to exude out to the surface of buffer members passing through the gaps in buffer members, or the adhesive is applied onto the surface of buffer members to establish a state where the adhesive is applied onto the surface of buffer members intermittently in the lengthwise direction, and the outer covering is provided thereon in a state where the adhesive has not yet been cured, so that the fiber core and buffer members are adhered together and that the outer covering and buffer members are adhered together intermittently in the lengthwise direction.